An airplane is propelled by one or several propeller assemblies comprising a turbojet engine housed in a tubular nacelle. Each propeller assembly is attached to the airplane by a mast situated generally under a wing or at the fuselage.
A nacelle generally has a structure comprising an air intake upstream from the engine, a middle section designed to surround a fan of the turbojet engine, and a downstream section housing thrust reverser means and intended to surround the combustion chamber of the turbojet engine, and generally ends with a jet nozzle whereof the outlet is situated downstream from the turbojet engine.
The air intake comprises, on one hand, an intake lip adapted to allow optimal collection toward the turbojet engine of the air necessary to supply the fan and the internal compressors of the turbojet engine, and on the other hand, a downstream structure on which the lip is attached and designed to suitably channel the air toward the blades of the fan. The assembly is attached upstream from a case of the fan belonging to the middle section of the nacelle.
The fan case is intended to surround the fan zone of the turbojet engine and ends upstream essentially at the vanes of the fan.
However, one should take into account the accidental case of the loss of a fan vane. To do this, the case is generally extended, via an extension called vane retention extension, until it forms a debris angle of about 15° (generally) relative to an attachment leg of said vane.
This extension defines the position of attachment flanges between the fan case and an inner panel of the air intake structure. Upstream from said interface, there is therefore no need to consider the apparent air intake structure as having to be capable of retaining the vane debris, which makes it possible to have lightened structure.
For acoustic reasons, it is sometimes necessary to avoid an acoustic panel rupture at the attachment flanges, as illustrated in document FR 2 847 304, and to extend the acoustic surface of the acoustic shroud of the air intake upstream of the vane heads above the vane retention extension, as illustrated in document FR 2 869 360, where an acoustic panel of the air intake structure extends until it covers said vane retention extension.
Document FR 2 898 870 also describes such a geometry.
However, in this case, the structure of the acoustic panel downstream from the attachment flanges is overhanging and can create significant aerodynamic disruptions (vibratory effects, uncontained geometric tolerances), which risks causing aeroacoustic disruptions more significant than the gain due to the attached acoustic surface.
Moreover, it is also necessary to take into account the provision of attachment flanges of the acoustic panel which, mounted on an outer skin of the acoustic panel for stress pick-up and structural maintenance reasons, requires having an extended interface and a sufficient structure thickness for the transit of the stresses. This has an impact on the mass of the assembly. These attachment flanges can also require supplementary structural reinforcement by rivets, which damages the acoustic response quality of the cells impacted by the housing of the bulbs of said rivets.